What is glass, how is it made?

Glass has been made by humans for about 4,500 years, and it generally consists of silica (sand), potassium or soda, lime, and occasionally some other chemicals. They are melted together and cooled at air temperature so that the material is hard, but has a smooth and flowing shape, more like a liquid. The precise recipe for glass changes from place to place — the chemistry and techniques are often closely guarded secrets.